The need has long existed for a device whereby both manufacturers and end users of a variety of alloys could conveniently identify metallic samples from which, for one reason or another, conventional identification markings are absent. Non-destructive testing methods which have been resorted to for such sorting purposes have relied on one or more of various physical characteristics of the metallic samples, including thermal properties, magnetic properties electrical properties, triboelectric properties, and thermoelectric properties. In the case of thermoelectric testing, use is made of the Seebeck effect whereby an electro-motive force (emf) is generated between two junctions of a dissimilar pair of metals when the junctions are at different temperatures.
One form of device for identifying metals by reliance on their Seebeck effect is described in U.S. Pat. No. 3,667,032. The device in questions features a pair of probes which are made of identical electrically conductive material. One of the probes is electrically heated to produce a predetermined temperature difference between the probes, and the emf between the hot and cold probes is measured. The use of probes made of the same material as one another eliminates the need to know precisely what the temperature of each probe is. However, it remains essential to know what the temperature difference between the probes is, with some accuracy, if the measured emf is to be relied upon for identification of the unknown metal.
An improvement on this technique which has been used by the assignee of the present invention for many years avoids the need for maintaining a precise temperature difference between the probes. This is achieved by relying not simply on the absolute value of the emf generated between the probes and the unknown sample, but on the relationship between that emf (which we will designate E.sub.x) and an electrical signal which is indicative of the temperature difference between the probes. This temperature indicating signal (which we will designate E.sub.s) can be derived for example from one or more thermocouples attached to the probe tips. Any change in the temperature difference between the probes will result in variations of both E.sub.x and E.sub.s. While in general the dependence of E.sub.x and E.sub.s on the temperature difference will not be identical, the difference in behavior can be ignored for minor changes in the temperature difference. Thus the ratio of E.sub.s to E.sub.x will be substantially constant despite minor variations in the temperature difference between the probes. In practice a potentiometric bridge circuit has been used to determine the above mentioned ratio obtained when a particular probe assembly is contacted with a series of known metals or alloys, and a chart was thereby constructed which enabled metallic specimens to be identified by means of the E.sub.s to E.sub.x ratio determined when the same probe assembly was contacted with the unknown specimen.
However, while the Seebeck effect measurement has proved a useful tool for metal sorting, and has indeed been used for this purpose by the assignee for many years, it has not been possible until now to design a reliable metal sorter having the desired degree of portability. Thus, whereas many devices are described in the literature as being portable, they are in fact cumbersome devices, requiring to be connected to a mains electrical supply or to some bulky power unit. In the known thermoelectric metal sorters, the probe head itself tends to be bulky because of the need to keep a substantial distance between the two probes in order to avoid the resistance heater surrounding the hot probe from also heating the cold probe. Other problems encountered in using such sorters arise from the need to establish good electrical and thermal contact between the probes and the unknown sample during the test. This requires the application of pressure to urge both probes against the sample, and the maintaining of such pressure while balancing a potentiometer or while reading a meter which may be fluctuating with the pressure applied to the probe head.